The best way of handling this I think would have been to take down the crib back in December to emphasize that it is no longer her room and she only has one bed- her toddler bed in the new room. And to praise her like crazy for being a big girl and sleeping in a big girl bed. But you already know that. Now it is kind of a tricky situation. Do you have a pack and play? If so, one thing you could do is put the baby down for naps in her new room. Explain to her that the baby is getting bigger and needs a bigger place to sleep. I would also mention..."Just like she did, she used to be so tiny like her brother and look at what a big girl she is now!" Keep mentioning that she is a big girl and a big sister and that she can do so many big girl things now. Help her list all of the things she can do that she couldn't do when she was a baby, and how proud you are. Having the baby in her room during the day might encourage her to take ownership of it, and "defend her territory" from the newcomer. This could backfire however since the baby is so new to the family and she is still adjusting, she may resent him for taking over and changing everything, as well as her space. You would have to take extra special care to keep a lot of things the same in your house, and make sure you have time alone together where she has your full attention to consistently let her know that even though her brother needs you a lot right now that she still means the world to you and that you appreciate her.
Another thing that might help is getting one of those tents that go over toddler beds, like a canopy type thing. Moving into a bed with no big sides and without an enclosed feeling might seem scary. Ask her what she likes best about the nursery, what are her favorite things about that room? It could be something that you could recreate in the new room to make it more comfortable.
You could also try having a few special "Girls only sleepovers" in there with her. Talk it up, and then sleep in there with her for a night or two. If you are breastfeeding this might be hard to do if your daughter is a light sleeper...but you could at least pretend to sleep and then leave the room when she is out cold. If you are formula feeding your son, have your husband take over for those couple of nights that you are in with your daughter. This will give you both some special time together and also get her used to being in the toddler bed. Another thing I would recommend if you haven't already done so is move her toys into the new room, and spend time playing in there together. Hang up any drawings she does on the wall in the new room. Put her laundry in there, and generally treat it as if she were already sleeping in there.
Bribery often works...think of some big thing she would love, and tell her if she moves into the big girl room she will get it. You could also give her small rewards for every night she sleeps in there...stickers are a classic way to record progress. Decorate a poster with her that says "Katie can do it!" Or "Katie is becoming a big girl" or something silly like that and for each night she sleeps in there she gets to put a different sticker up there. And for each 15 stickers that are up there, she gets a reward [like another sleepover with mommy].
If she still doesn't want to go in there, you might have to make the new room the new nursery for now...even if there are girly colors in there your son certainly will not care. Eventually once she adjusts to the new baby and feels secure again, she may change her mind about it.